1. Field of the Invention
The present invention relates to a method and an apparatus for defining the portion of wood material present in a bark flow coming out from a debarking process and for controlling the barking process on the basis of said data for reducing wood losses in the debarking process.
2. Related Art
In debarking, usually a barking process carried out by means of a debarking drum, the object is to remove bark from the surface of trees so as to achieve a desired degree of debarking. At the same time, however, there occurs grinding off and crushing of wood material itself, said wood material representing wood loss as it becomes part of a bark flow. Naturally, it is desirable the wood loss is kept at a minimum, especially since wood is a major cost factor in the production of pulp and paper. Generally, the bark flow may contain wood on the order from 10% to about 40, which equals wood losses of about 20% to about 50% the total quantity of wood material used.
In the prior art, the portion or share of wood in a bark flow is measured by picking up a sample from the bark flow. The sample is handled manually to separate wood and bark material from each other, followed by drying and then measuring the relative amount of wood. Drying is necessary in order to compare dry weights. According to SCAN-standard, the drying takes 16 hours.
The above type measurements have been used mostly for statistics, yet such measurements have had little significance in terms of process control as the situation could have changed during the time period incurred during drying of the sample.
An object of the invention is to provide a method and an apparatus for measuring wood loss in a debarking process essentially in real time and, thus, to provide a for controlling the debarking process for reducing wood losses.
According to the invention, this object is achieved and a method of the invention is characterized in that a bark flow coming out from debarking is measured optically for its whiteness and the measuring result is used as a basis for determining programmatically by means of a data processing unit the amount of wood material in the bark flow, and that the debarking process is controlled on the basis of the amount of wood material determined from the bark flow.
It should be stressed that, in this application, the term whiteness refers not only to various grey levels but also to color separation.
The apparatus of the invention is characterized in that a bark flow coming out from debarking is adapted to be imaged by means of a camera, said image taken by the camera being adapted to be processed with an image processing unit which uses different whitenesses of the picture elements or pixels of the image as a basis for determining programmatically through multiple inerations portions of wood material in the bark flow, and that the image processing unit is adapted to produce an output signal for controlling the debarking process.
The image is preferably processed in image sections, the particular picture elements of which may be varied between iterations.
The real-time measuring of a bark flow in accordance with the invention for determining the amount of wood material contained in the bark flow offers in an average wood room a possibility of saving about 10% to 20% of the total amount of wood, which represents about 5,000to about 40,000 solid cubic meters of wood annually, depending on the size of a wood room.
In addition, the invention relates to a method and an apparatus for determining the portion of wood material present in a bark flow being delivered to a combustion process and for controlling the combustion process on the basis of said data for optimizing the combustion process.
Especially in power plants of sawmills, pulp and paper mills, which burn a mixture of bark and wood material, a problem is the fluctuation of a heat value of the mixture to be burned. The most important factor effecting to the heat value is the moisture of a mixture to be burned, but also the fluctuation of the relative portions of e.g. bark and wood material in a mixture to be burned has an effect on the heat value.
In the method of the invention, the effect of fluctuation between the relative portions of bark and wood material upon the heat value is resolved in such a manner that from a bark flow is measured optically its whiteness and the measuring result is used as a basis for determining programmatically by means of a data processing unit the amount of wood material in the bark flow, and that the amount of wood material determined from the bark flow is used as a basis for calculating a heat value for the material present in the bark flow and for controlling the burning process as necessitated by said value.
The apparatus of the invention is characterized in that a bark flow is adapted to be imaged by means of a camera, said image taken by the camera being adapted to be processed with an image processing unit which uses different whitenesses of the picture elements of the image as a basis for determining programmatically the portion of wood material in the bark flow, and that the image processing unit is adapted to produce an output signal for controlling the burning process.